What can I say? Flying economy on all the major airlines sucks. United particularly galled me by not letting us reserve seat assignments in advance on the outbound flight, so my boyfriend and I ended up 7 rows apart.
One great tip for those flying back from Cancun. Use the airport assistance people (in light blue uniforms) to do the check in on the check in machines, and tip them. We gave our guy $5 and he managed to upgrade us to an exit row for no additional cost back to Newark. Best money we spent on the whole trip!